1. Field of the Invention
The present invention relates to sealing members for use in a gas preheater.
2. Related Art Statement
Gas preheaters are used for recovering heat from exhaust gases with combustion air through heat exchange in thermal power generating boilers, ship boilers, and chemical apparatuses such as oil refiners, evaporating kilns, and reforming furnaces, and for improving heat efficiency. FIG. 23 shows a perspective outline view of a principal portion of an air preheater.
A rotor 5 is rotatably placed in a rotor housing 30, and two radial sealing plates 31 are provided for each side face of the rotor 5. The rotor housing 30 is supported by pedestals not shown. The rotor 5 is rotated by a driving (unit not) shown in a direction of arrow H.
Hot exhaust gases are passed through an upper side of the rotor 5 as shown by an arrow D, and air is passed through a lower side of the rotor in a counter flow direction as shown by an arrow F. The exhaust gas layer is isolated from the air layer by the radial sealing plates 31. Heating elements, which are fitted into the rotor 5, absorb heat from the hot exhaust gases. When cold air is passed through the heating elements during rotation of the rotor 5, cold air is heated, and resulting hot air is then led to a boiler or the like (not shown) as shown by an arrow G. On the other hand, since heat is removed from the hot exhaust gas, cold exhaust gas is discharged outside as shown by an arrow E.
In such an air preheater, gaps are present between the outer periphery of the rotor 5 and the rotor housing 30 and between the side faces of the rotor 5 and the radial sealing plates 31, respectively. The exhaust gases and air leak through these gaps to lower heat efficiency. Therefore, it is important to enhance the sealing effect by making the gaps as small as possible.
The gap between the outer periphery of the rotor and the housing is structurally sealed by making this gap as small as possible.
The temperature of the exhaust gas is 300.degree.-400.degree. C., and that of air is in a range from room temperature to around 100.degree. C. The rotor in which the heating element is housed is deformed corresponding to changes in the above temperatures range. In the structural sealing, the gap becomes greater due to expansion or permanent strain owing to such temperature changes. Consequently, the sealing effect is deteriorated.
The gap between the side face of the rotor and the radial sealing plate is structurally sealed by making the gap between twelve o twenty four diaphragms attached to the side face of the rotor and the radial sealing plates made of a metal as small as possible.
However, such a technique has a limitation upon the sealing effect. When the structural material is corroded with an exhaust gas component, the gap becomes greater to lower the sealing effect.
On the other hand, when the gap between the sealing members is eliminated and the members are slid under pressure, it can be expected that the sealing effect is improved.
Besides the above-mentioned problems in the structural sealing, since a fuel used in a boiler is coal, heavy oil or the like in the case of the rotating type air preheater for the combustion in the boiler used in the thermal power station, corrosive gas components such as oxides of sulfur, oxides of nitrogen and the like are contained in exhaust gases. Thus, since the exhaust gas is condensed at lower temperature portions in the air preheater, there is the problem in that parts of the air preheater are likely to be corroded. Further, there is another problem in that dust in the hot exhaust gas attach to the parts of the heat elements of the air preheater, and deteriorate heat exchange efficiency of the air preheater. Thus, in order to remove such attached matters, the air preheater is timely washed with water. In addition, the boiler in the thermal power station is continuously operated for a long time period, and is stepped down for only a very short time. Therefore, the above attached matters are removed by washing the air preheater with water in a hot state at about 200.degree. C. without the air preheater being fully cooled. Consequently, there is another problem in that the parts of the air preheater undergo severe thermal shocks.
Furthermore, the rotary type air preheater to be used for the combustion of the boiler used in the thermal power station is a large scale apparatus in which the diameter of the rotor is as much as 1-20 m. Accordingly, the size of a single part to be used in the air preheater is great. For example, a sealing member having a size of 200.times.100.times.50 mm is used. Therefore, the thermal shock conditions against the sealing member are extremely severe in the case that the preheater is washed with water in the above hot state.
A slidable member to be used in the rotary type air preheater for combustion of boilers in the thermal power plant needs excellent wear resistance, corrosion resistance, and thermal shock resistance. Although SS steel or corrosion resistance steel is used as the conventional sealing member, wear resistance and corrosion resistance are still insufficient, so that troublesome maintenance is necessary. Particularly, such materials are not satisfactory as sliding members.